Plasma etching method and computer-readable storage medium

ABSTRACT

In a plasma etching method, a substrate, on which an oxide film as a target layer to be etched, a hard mask layer, and a patterned photoresist are sequentially formed, is loaded into the processing chamber and mounted on a lower electrode. A processing gas containing C x F y  (x is 3 or less and y is 8 or less), C 4 F 8 , a rare gas and O 2  is supplied and a plasma of the processing gas is generated by applying a high frequency power to an upper or a lower electrode. Further, a high frequency power for bias is applied to the lower electrode, and a DC voltage is applied to the upper electrode.

FIELD OF THE INVENTION

The present invention relates to a plasma etching method for performinga plasma etching on an oxide film formed on a substrate through a masklayer, and a computer-readable storage medium for storing therein acontrol program to be used in executing the plasma etching method.

BACKGROUND OF THE INVENTION

In a manufacturing process of a semiconductor device, a photoresistpattern is formed on a semiconductor wafer, which is a substrate to beprocessed, through a photolithography process to be used as a mask inetching of the semiconductor wafer.

With a recent trend of miniaturization of semiconductor devices, etchingalso requires microprocessing. To keep up with such trend ofmicro-etching, the thickness of a photoresist film used as a mask isgetting thinner, and the kind of the photoresist is shifting from a KrFphotoresist (i.e., a photoresist exposed to a laser beam of which anemission source is a KrF gas) to an ArF photoresist (i.e., a photoresistexposed to a laser beam having a shorter wavelength of which an emissionsource is an ArF gas) adequate for forming a pattern opening no greaterthan about 0.13 μm.

Since, however, the ArF photoresist has a low plasma resistance, itsuffers a surface roughening during an etching, which hardly occurs whenusing the KrF photoresist. Accordingly, there occur such problems of aformation of longitudinal strips (striation) on inner wall surfaces ofopenings, enlargements of openings (i.e., an increase of criticaldimension (CD)), and the like. As a result, due to the thin thickness ofthe ArF resist together with the above problems, it is difficult to formetching holes with a sufficient etching selectivity.

In order to solve the above problems, Japanese Patent Laid-openApplication No. 2006-41486 (Reference Document) discloses a techniqueinvolving the steps of forming an amorphous carbon film as a sacrificialhard mask on a target layer to be etched and forming thereon a patternedphotoresist film; etching the amorphous carbon by using the patternedphotoresist film as a mask; and then etching the target layer with atypically used CF-based gas by using at least the amorphous carbon filmas an etching mask. By this technique, problems concerning the etchingselectivity and the etching shapes can be solved to some extent.

However, in the etching of, e.g., a DRAM capacitor, it is required toform, on an oxide film, holes each having a very high aspect ratio witha width of about 80 nm and a depth of about 2 μm. Further, in nextgeneration capacitors, the width of the hole is required to be reducedless and less to, e.g., about 68 nm and further to, e.g., about 58 nm.With the technique of the Reference Document, however, it is difficultto form holes of such sizes in satisfactory shapes with a sufficientetching selectivity without suffering a bowing phenomenon and the like.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a plasmaetching method capable of obtaining a high etching selectivity and fineetching shapes when etching an oxide film to form micro holes eachhaving a high-aspect ratio therein.

Further, it is another object of the present invention to provide acomputer-readable storage medium for storing therein control programs tobe used in executing the plasma etching method.

In accordance with an aspect of the invention, there is provided amethod of a plasma etching method for plasma etching an oxide filmformed on a substrate through a hard mask layer by using a plasmaetching apparatus including a vacuum-evacuable processing chamberprovided with a lower electrode serving as a mounting table for asubstrate, and an upper electrode disposed to face the lower electrode,the plasma etching apparatus performing a plasma etching by converting aprocessing gas supplied in the processing chamber into a plasma by meansof applying a high frequency power of a relatively high frequency levelfor plasma generation to the upper or the lower electrode; applying ahigh frequency power of a relatively low frequency level for bias to thelower electrode; and also applying a DC voltage to the upper electrode,the method including the steps of: loading into the processing chamber asubstrate on which an oxide film as a target layer to be etched, a hardmask layer, and a patterned photoresist are formed in described order,and mounting the substrate on the lower electrode; supplying aprocessing gas containing C_(x)F_(y) (x is 3 or less and y is 8 orless), C₄F₈, a rare gas and O₂; generating a plasma of the processinggas by applying a high frequency power to the upper or the lowerelectrode; applying a high frequency power for bias generation to thelower electrode; and applying a DC voltage to the upper electrode.

In accordance with another aspect of the invention, there is provided amethod of a plasma etching method, which is performed by using a plasmaetching apparatus including a vacuum-evacuable processing chamberprovided with a lower electrode serving as a mounting table for asubstrate and an upper electrode disposed to face the lower electrode,the plasma etching apparatus performing a plasma etching by converting aprocessing gas supplied in the processing chamber into a plasma by meansof applying a high frequency power for both plasma generation and biasgeneration to the lower electrode; and also applying a DC voltage to theupper electrode,

the method including the steps of: loading into the processing chamber asubstrate on which an oxide film as a target layer to be etched, a hardmask layer, and a patterned photoresist are formed in described order,and mounting the substrate on the lower electrode; supplying aprocessing gas containing C_(x)F_(y) (x is an integer equal to or lessthan 3 and y is an integer equal to or less than 8), C₄F₈, a rare gasand O₂; generating a plasma of the processing gas and concurrentlyapplying a bias by applying a high frequency power for both plasmageneration and bias generation to the lower electrode; and applying a DCvoltage to the upper electrode.

The hard mask layer may be an amorphous carbon film. Further, theC_(x)F_(y) gas may be C₃F₈ or CF₄, and, when the C_(x)F_(y) gas is C₃F₈,it is preferable that the flow rate thereof is equal to or greater thanthe flow rate of C₄F₈. Moreover, the absolute value of the DC voltagepreferably ranges from about 800 to 1200 V. Further, the rare gas may beAr or Xe.

The plasma etching method of the present invention is particularlyeffective, when forming a hole having a width of about 70 to 90 nm andan aspect ratio of about 1:15 to 1:25.

In accordance with still another aspect of the invention, there isprovided a computer-readable storage medium for storing therein acomputer-executable control program for controlling a plasma etchingapparatus including a vacuum-evacuable processing chamber provided witha lower electrode serving as a mounting table for a substrate and anupper electrode disposed to face the lower electrode, the plasma etchingapparatus performing a plasma etching by converting a processing gassupplied in the processing chamber into a plasma by means of applying ahigh frequency power of a relatively high frequency level for plasmageneration to the upper or the lower electrode, and applying a highfrequency power of a relatively low frequency level for bias to thelower electrode; or applying a high frequency power for both plasmageneration and bias to the lower electrode; and also applying a DCvoltage to the upper electrode, wherein, when executed, the controlprogram controls the plasma etching apparatus to perform the plasmaetching method in the first or second aspect.

In accordance with the present invention, since a plasma etching isperformed on a substrate, on which an oxide film as a target layer to beetched, a hard mask layer and a patterned photoresist are sequentiallyformed, by a processing gas including C_(x)F_(y) (x is an integer equalto or less than 3, y is an integer equal to or less than 8), C₄F₈, arare gas and O₂, it is possible to form holes of fine etching shapeswithout bowing or the like at a practical etching rate, even if theholes have a high aspect ratio of a narrow width. Further, in aconventional process using the above processing gas system, a sufficientetching selectivity cannot be obtained so that the mask layer may beremoved before the etching is completed. However, in an etching processof the present invention, a high frequency power for plasma generationis applied to an upper electrode or a lower electrode, a DC power isapplied to the upper electrode when a plasma is formed. Accordingly,polymers are supplied onto the hard mask layer from the upper electrode,so that a plasma resistance of the hard mask layer is increased, therebyimproving the etching selectivity. As a result, fine etching can beperformed even with the processing gas system without having the masklayer removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of an embodiment given inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic cross sectional view showing an exemplary plasmaetching apparatus used in performing a plasma etching method inaccordance with the present invention;

FIG. 2 sets forth a circuit diagram to show a configuration of amatching unit connected to a first high-frequency power supply of theplasma etching apparatus shown in FIG. 1;

FIG. 3 depicts a cross sectional view to show a structure of asemiconductor wafer W used in the embodiment of the present invention;

FIGS. 4A and 4B provide schematic diagrams to show etching states of thestructure shown in FIG. 3;

FIG. 5 presents a schematic diagram to show a state in which a hard masklayer is removed during an etching of an oxide film;

FIG. 6 offers a schematic diagram to show a state in which the etchingof the oxide film is completed in accordance with the embodiment of thepresent invention;

FIG. 7 provides a diagram for the comparison of plasma states for bothcases of applying and not applying a DC voltage to an upper electrode;

FIG. 8 is a diagram showing a result of Experiment 1;

FIG. 9 is a diagram showing a result of the Experiment 1;

FIG. 10 is a diagram showing a result of the Experiment 1;

FIG. 11 is a diagram showing a result of Experiment 2;

FIG. 12 is a diagram showing a result of Experiment 3;

FIG. 13 is a diagram showing a result of Experiment 4;

FIG. 14 is a schematic diagram showing another type of plasma etchingapparatus applicable to the embodiment of the present invention; and

FIG. 15 is a schematic diagram showing still another type of plasmaetching apparatus applicable to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 provides a schematic cross sectional view to show an exemplaryplasma etching apparatus used in performing plasma etching methods inaccordance with the embodiment of the present invention.

The plasma etching apparatus is configured as a capacitively coupledparallel plate type plasma etching apparatus having a substantiallycylindrical chamber (processing vessel) 10 made of, e.g., aluminum ofwhich a surface is anodically oxidized. The processing chamber 10 isframe grounded.

A columnar susceptor support 14 is disposed at a bottom portion of thechamber 10 via an insulating plate 12 made of ceramic or the like.Further, a susceptor 16 made of, e.g., aluminum is disposed on thesusceptor support 14. The susceptor 16 serves as a lower electrode,while mounting thereon a substrate to be processed, e.g., asemiconductor wafer W.

Provided on top of the susceptor 16 is an electrostatic chuck 18 forattracting and holding the semiconductor wafer W with a help of anelectrostatic force. The electrostatic chuck 18 is structured to have anelectrode 20 made of a conductive film sandwiched between a pair ofinsulating layers or insulating sheets. A DC power supply 22 isconnected to the electrode 20. The semiconductor wafer W iselectrostatically attracted and held by the electrostatic chuck 18 tothe electrostatic force such as a Coulomb force generated by a DCvoltage applied from the DC power supply 22.

Further, disposed on the periphery of the top surface of the susceptor16 to surround the electrostatic chuck 18 (semiconductor wafer W) is afocus ring (calibration ring) 24 made of, e.g., silicon, for improvingetching uniformity. A cylindrical inner wall member 26 made of, e.g.,quartz is disposed on lateral surfaces of the susceptor 16 and thesusceptor support 14.

A coolant passage 28 is provided inside the susceptor support 14circumferentially, for example. A coolant, e.g., cooling water, of aspecific temperature is supplied from a chiller unit (not shown) locatedat outside into the coolant passage 28 through lines 30 a and 30 b to becirculated therein, whereby a processing temperature of thesemiconductor wafer W on the susceptor 16 can be controlled bycontrolling the temperature of the coolant.

Moreover, a thermally conductive gas, e.g., He gas, is supplied from athermally conductive gas supply unit (not shown) into a space betweenthe top surface of the electrostatic chuck 18 and the backside of thesemiconductor wafer W through a gas supply line 32.

An upper electrode 34 is installed above the susceptor 16 serving as thelower electrode, to face the susceptor 16 in parallel. A space betweenthe upper and lower electrodes 34, 16 becomes a plasma generation space.The upper electrode 34 forms a facing surface, i.e., a surface being incontact with the plasma generation space while facing the semiconductorwafer W on the susceptor 16.

The upper electrode 34 is held by an insulating shield 42 at a ceilingportion of the chamber 10. The upper electrode 34 includes an electrodeplate 36 and an electrode support 38. The electrode plate 36 forms thefacing surface to the susceptor 16 and is provided with a plurality ofinjection openings 37. The electrode support 38 holds the electrodeplate 36 such that the electrode plate 36 can be detachably attached tothe electrode support 38. The electrode support 38 of a water coolingtype is made of a conductive material, e.g., aluminum of which thesurface is anodically oxidized. Preferably, the electrode plate 36 is alow-resistance conductor or semiconductor of a low Joule heat.Meanwhile, in order to strengthen a photoresist, the electrode plate 36is preferably made of a material containing silicon. Thus, the electrodeplate 36 is preferably made of silicon or SiC. A gas diffusion space 40is provided in the electrode support 38. A plurality of gas holes 41extends downwards from the gas diffusion space 40 to communicate withthe gas injection openings 37 A gas inlet opening 62 is formed in theelectrode support 38 to introduce a processing gas into the gasdiffusion space 40. A gas supply line 64 is connected to the gas inletopening 62, and a processing gas supply source 66 is connected to thegas supply line 64. A mass flow controller (MFC) 68 and aclosing/opening valve 70 are sequentially provided from the upstreamside in the gas supply line 64 (here, an FCS can be used instead of theMFC). Further, a processing gas containing C_(x)F_(y) (x is an integerequal to or less than 3 and y is an integer equal to or less than 8),C₄F₈ and O₂ is supplied from the processing gas supply source 66 intothe gas diffusion space 40 via the gas supply line 64 to be finallyinjected into the plasma generation space in a shower shape through thegas holes 41 and the gas injection openings 37. That is, the upperelectrode 34 functions as a shower head for supplying the processinggas.

A first high-frequency power supply 48 is electrically connected to theupper electrode 34 via a matching unit 46 and a power supply rod 44. Thefirst high-frequency power supply 48 outputs a high frequency power of10 MHz or higher, e.g., about 60 MHz. The matching unit 46 matches aload impedance to an internal (or output) impedance of the firsthigh-frequency power supply 48, and serves to render the outputimpedance of the first high-frequency power supply 48 and the loadimpedance be seemingly matched to each other when a plasma is generatedin the chamber 10. An output terminal of the matching unit 46 isconnected to the top end of the power supply rod 44.

Meanwhile, a variable DC power supply 50, as well as the firsthigh-frequency power supply 48, is electrically connected to the upperelectrode 34. The variable DC power supply 50 may be a bipolar powersource. Specifically, the variable DC power supply 50 is connected tothe upper electrode 34 via the matching unit 46 and the power supply rod44. The power feed of the variable DC power supply 50 can be controlledby an on/off switch 52. The polarity, current and voltage of thevariable DC power supply 50 and the on/off operation of the on/offswitch 52 are controlled by a controller 51.

As shown in FIG. 2, the matching unit 46 has a first variable capacitor54 and a second variable capacitor 56, and functions as described aboveby using the first and second variable capacitors 54 and 56. The firstvariable capacitor 54 is branched from a power feed line 49 of the firsthigh-frequency power supply 48, and the second variable capacitor 56 isprovided at a downstream side of the branching point in the power feedline 49. Further, a filter 58 is provided in the matching unit 46 totrap a high frequency (e.g., 60 MHz) from the first high-frequency powersupply 48 and a high frequency (e.g., 2 MHz) from a secondhigh-frequency power supply to be described later, thus allowing a DCvoltage current (hereinafter, referred to as “DC voltage”) to beefficiently supplied to the upper electrode 34. That is, the variable DCpower supply 50 is connected through the filter 58 to the power feedline 49. The filter 58 includes a coil 59 and a capacitor 60, and thehigh frequency from the first high-frequency power supply 48 and thehigh frequency from the second high-frequency power supply are trappedby the coil 59 and the capacitor 60.

A cylindrical ground conductor 10 a extends upwards from a sidewall ofthe chamber 10 to be located at a position higher than the upperelectrode 34. The ceiling wall of the cylindrical ground conductor 10 ais electrically insulated from the power supply rod 44 by a tubularinsulation member 44 a.

The second high-frequency power supply 90 is electrically connectedthrough a matching unit 88 to the susceptor 16 serving as the lowerelectrode. When a high-frequency power is supplied from the secondhigh-frequency power supply 90 to the susceptor 16, ions are attractedto the semiconductor wafer W. The second high-frequency power supply 90outputs a high frequency power of a range from 300 KHz to 13.56 MHz,e.g., 2 MHz. The matching unit 88 matches a load impedance to aninternal (or output) impedance of the second high-frequency power supply90, and renders the internal impedance of the second high-frequencypower supply 90 and the load impedance be seemingly matched to eachother when a plasma is generated in the chamber 10.

A low pass filter (LPF) 92 is electrically connected to the upperelectrode 34 for passing the high frequency (e.g., 2 MHz) from thesecond high-frequency power supply 90 to the ground, without allowingthe high frequency (e.g., 60 MHz) from the first high-frequency powersupply 48 to pass therethrough. Although the LPF 92 preferably includesan LR filter or an LC filter, it may include a single conducting wirecapable of applying sufficiently high reactance to the high frequency(60 MHz) from the first high-frequency power supply 48. Meanwhile,electrically connected to the susceptor 16 is a high pass filter (HPF)94 for passing the high frequency (60 MHz) from the first high-frequencypower supply 48 to the ground.

A gas exhaust port 80 is provided in the bottom of the chamber 10, and agas exhaust unit 84 is connected to the gas exhaust port 80 through agas exhaust line 82. The gas exhaust unit 84 has a vacuum pump such as aturbo-molecular pump, and can depressurize the inside of the chamber 10to a desired vacuum level. Further, a loading/unloading port 85, throughwhich the semiconductor wafer W is loaded and unloaded, is provided inthe sidewall of the chamber 10. The loading/unloading port 85 can beopened and closed by a gate valve 86. Further, a deposition shield 11 isdetachably installed at the inner wall of the chamber 10 so as toprevent etching byproducts (deposits) from being attached to the chamber10. That is, the deposition shield 11 serves as a chamber wall. Thedeposition shield 11 is also provided on the outer surface of the innerwall member 26. A gas exhaust plate 83 is provided at a lower portion ofthe chamber 10 between the deposition shield 11 installed at the innerwall of the chamber 10 and the deposition shield 11 disposed at theinner wall member 26. The deposition shield 11 and the gas exhaust plate83 can be appropriately formed by covering an aluminum material withceramic such as Y₂O₃.

Further, a conductive member (GND block) 91 DC-connected to the groundis provided to a portion of the deposition shield 11 forming the chamberinner wall at a height position substantially identical with the heightof the wafer W. With this configuration, an abnormal discharge can beprevented.

Each component of the plasma etching apparatus is connected to andcontrolled by a control unit (for controlling the whole components) 95.Further, a user interface 96 is connected to the control unit 95,wherein the user interface 96 includes, e.g., a keyboard for a processmanager to input a command to operate the plasma processing apparatus, adisplay for showing an operational status of the plasma processingapparatus and the like.

Moreover, connected to the control unit 95 is a storage unit 97 forstoring therein, e.g., control programs to be used in realizing variousprocesses, which are performed in the plasma processing apparatus underthe control of the control unit 95 and programs or recipes to be used inoperating each component of the plasma processing apparatus to carry outprocesses in accordance with processing conditions. The recipes can bestored in a hard disk or a semiconductor memory, or can be set at acertain position of the storage unit 97 while being recorded on aportable storage medium such as a CDROM, a DVD and the like.

When a command or the like is received from the user interface 96, thecontrol unit 95 retrieves a necessary recipe from the storage unit 97and executes the recipe. Accordingly, a desired process is performed inthe plasma processing apparatus under the control of the control unit95.

Hereinafter, there will be described a plasma etching method inaccordance with a first embodiment of the present invention, which isperformed by the plasma etching apparatus having the aforementionedconfiguration.

Referring to FIG. 3, a semiconductor wafer W to be processed has anetching stop film 102, an oxide film 103 as a target layer to be etched,a hard mask layer 104, a bottom anti-reflection coating (BARC) film 105and a photoresist film 106 that are sequentially formed on a Sisubstrate 101, wherein the photoresist film is provided with a certainpattern. The hard mask layer 104 is first etched by using thephotoresist film 106 as a mask, and the oxide film 103 which is a targetlayer is then etched.

In this embodiment, the oxide film 103 can be formed of, e.g.,tetraethoxysilane (TEOS), a glass film (BPSG or PSG) or the like. Thethickness of the oxide film 103 is appropriately set, and, for example,when it is used as a DRAM capacitor, its thickness is set to be in arange from about 1.5 to 3.0 μm.

As the hard mask layer 104, an amorphous carbon film can beappropriately utilized. The amorphous carbon film has the same level ofplasma resistance as those of SiN and SiC, which have been typicallyused as the hard mask layer, and it also has a low price. Here, it ispossible to use such a typically employed material as TiN, SiN or thelike instead of the amorphous carbon. The thickness of the hard masklayer 104 ranges from about 500 to 900 nm.

The etching stop film 102 is made of a SiC-based material such as SiCN,and its thickness ranges from about 20 to 100 nm. The BARC film 105 is aSiON film or an organic film, and its thickness is about 20 to 100 nm.The photoresist film 106 is typically an ArF resist of which a thicknessranges from about 100 to 400 nm.

In an etching processing, the gate valve 86 is first opened, and thesemiconductor wafer W having the above-described configuration is loadedinto the chamber 10 through the loading/unloading port 85 to be mountedon the susceptor 16. Then, a processing gas for the etching is suppliedfrom the processing gas supply source 66 into the gas diffusion space 40at a predetermined flow rate and is then supplied into the chamber 10via the gas holes 41 and the gas injection openings 37. While theprocessing gas being supplied into the chamber 10, the chamber 10 isevacuated by the gas exhaust unit 84 so that the internal pressure ofthe chamber 10 is maintained at a set value within a range from, e.g.,about 20 to 30 Pa. Further, a susceptor temperature is set to be in arange from about 20 to 50° C.

Here, a gas containing C_(x)F_(y) (x is an integer equal to or less than3 and y is an integer equal to or less than 8), C₄F₈, a rare gas and O₂is used as a processing gas for etching the oxide film 103. Though theprocessing gas may additionally contain other gases, it is preferable tocompose the processing gas only with these four gases. The C₄F₈ gasfunctions to facilitate a vertical formation of mask shapes and, for thereason, it is an important gas for improving etching shapes. However, ifonly the C₄F₈ gas is supplied at a flow rate capable of obtaining asufficient etching rate, deposits are formed on inner peripheralsurfaces of etching holes, thus increasing the probability that adefective shape such as a bowing is generated in a next etching step.For the reason, by using the C_(x)F_(y) gas in which x is 3 or less andy is 8 or less, which contains smaller amount of C per a molecule thanthe C₄F₈ gas, the deposits can be reduced. In order to reduce thegeneration of the deposits more effectively, it is preferable toincrease the temperature of the susceptor 16, i.e., the lower electrode,up to about 50° C.

As the C_(x)F_(y) gas, C₃F₈ or CF₄ can be appropriately employed.Particularly, C₃F₈ is preferable. The C₃F₈ has a function of increasingan etching rate. When using the C₃F₈, its flow rate is preferably set tobe higher than that of the C₄F₈. By setting as such, it is possible toeffectively suppress deposits from being formed in openings. Morepreferably, a flow rate ratio between the C₃F₈ and the C₄F₈ is set to beabout 1:1 to 1.5:1. Specifically, the flow rate of the C₃F₈ ispreferably set to be in a range from about 20 to 60 mL/min (flow rateconversed in a standard state (sccm)), and the flow rate of the C₄F₈ ispreferably set to be in a range from about 20 to 40 mL/min (sccm).

The O₂ gas is used to enlarge a bottom CD (Critical Dimension) of anetching hole by obtaining an etch profile (capability to form deep holeswithout suffering an etch stop) thereof and to secure a balance of theprocessing gas. It is preferable to add the O₂ gas such that a flow ratepercent of the O₂ gas to the entire processing gas is about 2.5 to 3.5%.To be specific, the flow rate of the O₂ gas is preferably set to bewithin a range from about 20 to 30 mL/min (sccm).

The rare gas is added to obtain the etch profile of the etching hole andto obtain a balance of the processing gas by diluting the CF-based gas,to thereby control deposits or fluorine F. It is preferable to add therare gas such that a flow rate percent of the rare gas to the entireprocessing gas is about 85 to 90%. Specifically, the flow rate of therare gas is preferably set to be in a range from about 600 to 900 mL/min(sccm). Here, the flow rate of the rare gas depends on the material ofthe hard mask layer 104. In case the hard mask layer 104 is amorphouscarbon, the flow rate of the rare gas is preferably set to be about 800mL/min (sccm) or higher. However, if the hard mask layer 104 is formedof a PolyMask material, it is preferable to set the flow rate of therare gas to be about 300 mL/min (sccm) or less.

As a rare gas, Ar or Xe can be appropriately employed. In particular, byusing the Xe as the rare gas, the function as a carrier of C can beenhanced, whereby linearity of the etching can be improved. As a result,fine etching shapes of the hard mask layer 104 and the oxide film 103can be obtained.

The etching of the hard mask layer 104, which is carried out prior toetching of the oxide film 103, is performed under typical processingconditions. For example, in case the hard mask layer 104 is amorphouscarbon, a gas containing, e.g., C₄F₆, a rare gas (Ar) and O₂ is used asa processing gas.

After the processing gas for the etching is introduced into the chamber10, a high frequency power for plasma generation is applied from thefirst high-frequency power supply 48 to the upper electrode 34 at aspecific power level, and, at the same time, a high frequency power forion attraction is applied from the second high-frequency power supply 90to the susceptor 16, i.e., the lower electrode, at a certain powerlevel. Further, a DC voltage is applied from the variable DC powersupply 50 to the upper electrode 34. Moreover, a DC voltage is appliedfrom the DC power supply 22 to the electrode 20 of the electrostaticchuck 18, whereby the semiconductor wafer W is firmly fixed on thesusceptor 16.

The processing gas injected from the gas injection openings 37 formed inthe electrode plate 36 of the upper electrode 34 is converted into aplasma by a glow discharge generated between the upper electrode 34 andthe susceptor 16 serving as the lower electrode by the high frequencypowers applied thereto. By radicals or ions generated from the plasma,the hard mask layer 104 is first etched by using the photoresist film106 as a mask, so that the resist pattern is transcribed to the hardmask layer 104, as shown in FIG. 4A. Thereafter, the oxide film 103 isetched by using the hard mask layer 104 as a mask, thereby obtaining ahole 107, as illustrated in FIG. 4B.

Since the high frequency power of a high frequency range (e.g., 10 MHzor higher) is applied to the upper electrode 34, the plasma can begenerated at a high density in a desirable state, and so it becomespossible to form a high-density plasma even under a lower pressurecondition.

However, if the etching of the oxide film is performed with theabove-mentioned processing gas only by applying the high frequencypowers, an etching selectivity of the oxide film to the hard mask layer104 is low while fine etching shapes can be obtained. As a result,before the etching of the oxide film 103 is completed, the hard masklayer 104 would disappear as shown in FIG. 5.

Thus, in this embodiment, when generating the plasma, a DC voltagehaving a specific polarity and magnitude is applied to the upperelectrode 34 from the variable DC power supply 50. By appropriatelycontrolling the DC voltage, a fine selectivity to the hard mask layer104 can be obtained, so that it becomes possible to etch the oxide film103 in a good shape in a state where the hard mask 104 still remains, asillustrated in FIG. 6. At this time, the absolute value of the DCvoltage is preferably set to be in a range of from about 800 to 1200 V.

This will be explained in further detail.

Polymers are attached at the upper electrode 34 during the prior etchingprocess, particularly an etching process in which a high frequency powerof a low level is applied to the upper electrode 34. If a proper DCvoltage is applied to the upper electrode 34 when performing an etchingprocess, a self bias voltage V_(dc) of the upper electrode 34 can bemade higher, that is, the absolute value of the V_(dc) at the surface ofthe upper electrode 34 can be increased, as shown in FIG. 7. As aresult, the polymers attached at the upper electrode 34 are sputtered bythe applied DC voltage and are supplied to the semiconductor wafer W tobe deposited on the hard mask layer 104. Thus, the etching of the hardmask layer 104 becomes difficult, so that the oxide film 103 can beetched with a high selectivity.

Moreover, when etching the oxide film 103, if the DC voltage is appliedto the upper electrode 34, electrons generated in the vicinity of theupper electrode 34 when the plasma is generated are accelerated in avertical direction in the processing space. At this time, by controllingthe DC voltage appropriately, the electrons can be made to reach theinside of vias, so that a shading effect can be suppressed, and betterhole shapes can be obtained.

Further, if the DC voltage is applied to the upper electrode 34 when theplasma is generated, a plasma density at a relatively central region ofthe chamber 10 can be increased, due to the diffusion of the plasma.When the internal pressure of the chamber 10 is comparatively high and anegative gas such as a CF-based gas is used as a processing gas, theplasma density at the central region of the chamber 10 tends to be low.In such case, by increasing the plasma density at the central region ofthe chamber 10 by the application of the DC voltage, a uniform plasmadensity can be attained.

Furthermore, it is possible to obtain a sufficient etching selectivityonly with the photoresist film without using the hard mask layer byselecting conditions for a rich deposition by a DC voltage application.In such case, however, deposits would be attached on the innerperipheral surfaces of etching holes, causing bowing or taperingthereof. For the reason, the use of the hard mask layer 104 isessential.

Below, experimental results for investigating the effects of the etchingmethod in accordance with the embodiment of the present invention willbe described.

(Experiment 1)

A sample used in this experiment was fabricated to have a structureshown in FIG. 3, by sequentially forming, on a Si substrate, a SiN filmhaving a thickness of 50 nm as an etching stop film 102, a two-layeredfilm formed of a BPSG film (lower layer) and a TEOS film (upper layer)having a thickness 1500 nm as a target oxide film 103 to be etched, anamorphous carbon film having a thickness of 500 nm as a hard mask layer104, a SiON film having a thickness of 60 nm as a bottom anti-reflectioncoating (BARC) film 105, and an ArF resist having a thickness of 200 nmas a photoresist film 106. After etching the hard mask layer 104 byusing the apparatus shown in FIG. 1, the oxide film 103 was etched undervarious conditions by using residues of the photoresist film 106 and thehard mask layer 104 as an etching mask. Here, etching of circular holeseach having a diameter of 90 nm was performed.

For the etching of the oxide film, processing conditions were asfollows: an internal chamber pressure was 2.7 Pa, a high frequency powerto an upper electrode was 1200 W, a high frequency power to a lowerelectrode was 3800 W, a DC voltage was −1000 V, an upper electrodetemperature was 95° C., a lower electrode temperature was 10° C., andC₃F₈, C₄F₈, Ar and O₂ were used as a processing gas, wherein the etchingwas performed by varying the flow rates of the C₃F₈, C₄F₈, Ar and O₂.

First, the etching was performed by varying the flow rates of the C₃F₈and the C₄F₈, while maintaining the flow rates of the Ar and the O₂ at800 mL/min (sccm) and 25 mL/min (sccm), respectively. Etching shapesobtained at that time are shown in FIG. 8. FIG. 8 shows etching shapesobtained by setting the flow rates of the C₄F₈ and the C₃F₈ as follows.

A) C₄F₈: 35 mL/min (sccm), C₃F₈: 30 mL/min (sccm)

B) C₄F₈: 30 mL/min (sccm), C₃F₈: 35 mL/min (sccm)

C) C₄F₈: 25 mL/min (sccm), C₃F₈: 40 mL/min (sccm)

D) C₄F₈: 20 mL/min (sccm), C₃F₈: 45 mL/min (sccm)

As shown in FIG. 8, C shows best shapes of shoulder portions ofopenings.

Then, the flow rates of C₃F₈ and C₄F₈ were fixed at the values in C ofFIG. 8, and the flow rates of the Ar and the O₂ were varied, while theother processing conditions were identical with those for the priorexperiment.

Etching shapes obtained at that time are illustrated in FIG. 9. FIG. 9shows etching shapes obtained by setting the flow rates of the Ar andthe O₂ as follows.

E) Ar: 500 mL/min (sccm), O₂: 34 mL/min (sccm)

F) Ar: 700 mL/min (sccm), O₂: 32 mL/min (sccm)

G) Ar: 900 mL/min (sccm), O₂: 30 mL/min (sccm)

H) Ar: 1100 mL/min (sccm), O₂: 28 mL/min (sccm)

Among them, G shows best shapes of shoulder portions of openings.Further, as a result of performing a process tuning by changing the gasratio for a center rich, satisfactory shapes without suffering bowingscould be obtained, as illustrated in FIG. 10.

At that time, a top CD, a middle CD once suffering bowings, a bottom CDwere of satisfactory values as 89 nm, 89 nm and 74 nm at a wafer centerregion, respectively; as 91 nm, 93 nm, 75 nm at a wafer middle region,respectively; and as 85 nm, 87 nm and 73 nm at a wafer edge region,respectively.

From the above results, it was confirmed that etching shapes can beimproved by setting a certain condition in which the flow rates of theC₃F₈ gas and the Ar gas are high.

(Experiment 2)

Here, a sample having the same structure as that of the Experiment 1 wasfabricated, and after etching a hard mask layer 104 by using theapparatus shown in FIG. 1, an oxide film 103 was etched by usingresidues of a photoresist film 106 and the hard mask layer 104 as amask. In this experiment, a processing pressure, an upper electrodetemperature and a lower electrode temperature were maintained at 2.7 Pa,95° C. and 10° C., respectively.

Further, in condition I, a high frequency power to the upper electrodeand a high frequency power to the lower electrode were set to be 1200 Wand 3800 W, respectively; a DC voltage was set to be −1000 V; and flowrates of C₄F₈, C₃F₈, Ar and O₂ were set to be 40 mL/min (sccm), 25mL/min (sccm), 900 mL/min (sccm) and 30 mL/min (sccm), respectively.

In condition J, the high frequency power to the upper electrode and thehigh frequency power to the lower electrode were set to be 1200 W and3800 W, respectively; the DC voltage was set to be −1000 V; and the flowrates of C₄F₈, C₃F₈, Ar and O₂ were set to be 25 mL/min (sccm), 40mL/min (sccm), 1000 mL/min (sccm) and 28 mL/min (sccm), respectively.

In condition K, the high frequency power to the upper electrode and thehigh frequency power to the lower electrode were set to be 1500 W and4500 W, respectively; the DC voltage was set to be −1100 V; and the flowrates of C₄F₈, C₃F₈, Ar and O₂ were set to be 25 mL/min (sccm), 40mL/min (sccm), 1000 mL/min (sccm) and 25 mL/min (sccm), respectively.

FIG. 11 provides the results. As shown in the figure, if the processingcondition shifts from the condition I to the condition J in which aratio of the flow rate of the C₃F₈ to the flow rate of the C₄F₈increases, etching shapes become improved. Further, if the processingcondition shifts from the condition J to the condition K in which theupper and the lower electrode powers and the DV voltage are increasedwhile the flow rate of the O₂ is reduced, CDs become shrunk, so thateven better etching shapes can be obtained.

(Experiment 3)

Here, a sample was fabricated to have the structure shown in FIG. 3, bysequentially forming, on a Si substrate 101, a SiN film having athickness of 40 nm as an etching stop film 102, a PSG film having athickness of 2.0 μm as a target oxide film 103 to be etched, anamorphous carbon film having a thickness of 400 nm as a hard mask layer104, a SiON film having a thickness of 60 nm as a BARC film 105 and anArF resist having a thickness of 200 nm as a photoresist film 106. Afteretching the hard mask layer 104 by using the apparatus shown in FIG. 1,the oxide film 103 was etched under various conditions by using residuesof the photoresist film 106 and the hard mask layer 104 as an etchingmask.

Here, etching of elliptical holes each having a longer diameter of 160nm and a shorter diameter of 80 nm was performed with an aspect ratio ofabout 25. For the etching of the oxide film, processing conditions wereas follows: an internal chamber pressure was 3.3 Pa, a high frequencypower to the upper electrode was 1000 W, a high frequency power to thelower electrode was 4500 W, a DC voltage was −500 V, an upper electrodetemperature was 95° C., a lower electrode temperature was 50° C., andC₃F₈, C₄F₈, Xe and O₂ were used as a processing gas, wherein the etchingwas performed by fixing the flow rate of the Xe gas at 400 mL/min (sccm)while varying the flow rates of the other gases.

In condition L, the flow rates of the C₄F₈, the C₃F₈, and the O₂ wereset to be 20 mL/min (sccm), 20 mL/min (sccm) and 12.5 mL/min (sccm),respectively (i.e., a ratio of C₃F₈/C₄F₈ was set to be 1). In conditionM, the flow rates of the C₄F₈, the C₃F₈, and the O₂ were set to be 10mL/min (sccm), 30 mL/min (sccm) and 10 mL/min (sccm), respectively(i.e., a ratio of C₃F₈/C₄F₈ was set to be 3). In condition N, the flowrates of the C₄F₈, the C₃F₈, and the O₂ were set to be 6.7 mL/min(sccm), 33.3 mL/min (sccm) and 7.5 mL/min (sccm), respectively (i.e., aratio of C₃F₈/C₄F₈ was set to be about 5).

FIG. 12 shows the results. As can be seen from the figure, if the ratioof C₃F₈/C₄F₈ increases from 1 to 3, bowings of holes at a wafer edgeportion are ameliorated conspicuously. When the ratio of C₃F₈/C₄F₈ was5, however, widths of holes at a wafer center portion are enlargedremarkably, though bowing thereat almost disappears. Further, it wasconfirmed that the etching selectivity becomes decreased by increasingthe ratio of C₃F₈ to C₄F₈. Thus, as the ratio of C₃F₈ to C₄F₈ increases,etching holes suffers bowing, bowingless and width enlargementsequentially. When the shape difference at the center and the edgeportions are considered along with etching selectivity, it is confirmedthat an optimum ratio of C₃F₈/C₄F₈ is 3.

Here, it is to be noted that the ratio of C₃F₈/C₄F₈ capable ofameliorating the bowing shape may depend on a thickness and hardness ofthe hard mask layer, a hardness of the oxide film and a ratio between ashorter diameter and a longer diameter of a hole.

(Experiment 4)

In this experiment, etching uniformity with an application of a DCvoltage was investigated.

Here, a sample was fabricated by sequentially forming, on a Si substrate101, a SiN film having a thickness of 60 nm as an etching stop film 102,a BPSG film having a thickness of 2000 nm as a target oxide film 103 tobe etched, forming a SiON film having a thickness of 60 nm as a BARCfilm 105, and an ArF resist having a thickness of 650 nm as aphotoresist film 106. This structure is obtained by omitting the hardmask layer 104 from the structure illustrated in FIG. 3. Then, by usingthe apparatus shown in FIG. 1, the oxide film 103 was etched undervarious conditions by using the photoresist film 106 as an etching mask.As a processing gas, C₄F₆, CF₄, Ar and O₂ were used, wherein their flowrates were set to be 40 mL/min (sccm), 60 mL/min (sccm), 350 mL/min(sccm) and 45 mL/min (sccm), respectively. A processing pressure was setto be 2.67 Pa (20 mTorr), and an etching rate and an etching selectivitywere measured while varying a high frequency power to the upperelectrode and a DC voltage.

FIG. 13 shows the results. As can been seen from the figure, if the DCvoltage is increased, a hole etching rate at a wafer center portionbecomes increased, while a hole etching rate at a wafer edge portionbecomes increased if the high frequency power to the upper electrode isincreased. From this, it was confirmed that a wafer in-surface etchingrate can be controlled by adjusting the DC voltage applied to the upperelectrode or the high frequency power to the upper electrode. Further, areversal of center/edge etching rates can also be performed readily.

It is to be noted that the present invention is not limited to theembodiment described above, but it can be modified in various ways. Forexample, though the embodiment has been described for the case of usingamorphous carbon as the hard mask layer, conventionally used othermaterials as exemplified above may be utilized instead. Further, thoughthe oxide film is described to be made of TEOS, BPSG, or PSG, thematerial for the oxide film is not limited thereto.

In addition, the apparatus to which the present invention is applied isnot limited to the one shown in FIG. 1, either. For example, it ispossible to use a type in which an upper electrode is divided into acentral part and a peripheral part so that high frequency powers appliedthereto can be controlled individually. Further, it is also possible touse a plasma etching apparatus of a type in which dual frequency powersare applied to a lower electrode. In this type of apparatus, a highfrequency power of, e.g., about 40 MHz for plasma generation is appliedfrom a first high-frequency power supply 48′ to the susceptor 16 whichserves as the lower electrode, and a second high frequency power of,e.g., about 2 MHz for ion attraction is concurrently applied to thesusceptor 16 from a second high-frequency power supply 90′. As shown inthe figure, by connecting the a variable DC power supply 166 to an upperelectrode 234 and applying a DC voltage thereto, the same effects asobtained in the above embodiments can be achieved.

Furthermore, as illustrated in FIG. 15, it is also possible to use anetching apparatus of a type having a high-frequency power supply 170instead of the first high frequency power supplies 48′ and the secondhigh-frequency power supply 90′ connected to the susceptor 16 in FIG.14. In such case, a high frequency power of, e.g., about 40 MHz for bothplasma generation and bias is applied to the susceptor 16, i.e., thelower electrode. As in FIG. 14, by connecting a variable DC power supply166 to an upper electrode 234 and applying a specific DC voltagethereto, the same effects as obtained in the above experiments can beachieved.

While the invention has been shown and described with respect to theembodiments, it is understood by those skilled in the art that variouschanges and modifications may be made without departing from the scopeof the invention as defined in the following claims.

1. A plasma etching method for plasma etching an oxide film formed on asubstrate through a hard mask layer by using a plasma etching apparatusincluding a vacuum-evacuable processing chamber having therein a lowerelectrode serving as a mounting table for a substrate and an upperelectrode disposed to face the lower electrode, the plasma etchingapparatus performing a plasma etching by converting a processing gassupplied in the processing chamber into a plasma by means of applying ahigh frequency power of a relatively high frequency level for plasmageneration to the upper or the lower electrode; applying a highfrequency power of a relatively low frequency level for bias to thelower electrode; and also applying a DC voltage to the upper electrode,the method comprising the steps of: loading into the processing chambera substrate on which an oxide film as a target layer to be etched, ahard mask layer, and a patterned photoresist are sequentially formed,and mounting the substrate on the lower electrode; supplying aprocessing gas containing C_(x)F_(y) (x is an integer equal to or lessthan 3 and y is an integer equal to or less than 8), C₄F₈, a rare gasand O₂; generating a plasma of the processing gas by applying a highfrequency power to the upper or the lower electrode; applying a highfrequency power for bias to the lower electrode; and applying a DCvoltage to the upper electrode.
 2. A plasma etching method, which isperformed by using a plasma etching apparatus including avacuum-evacuable processing chamber having therein a lower electrodeserving as a mounting table for a substrate and an upper electrodedisposed to face the lower electrode, the plasma etching apparatusperforming a plasma etching by converting a processing gas supplied inthe processing chamber into a plasma by means of applying a highfrequency power for both plasma generation and bias to the lowerelectrode; and also applying a DC voltage to the upper electrode, themethod comprising the steps of: loading into the processing chamber asubstrate on which an oxide film as a target layer to be etched, a hardmask layer, and a patterned photoresist are sequentially formed, andmounting the substrate on the lower electrode; supplying a processinggas containing C_(x)F_(y) (x is an integer equal to or less than 3 and yis an integer equal to or less than 8), C₄F₈, a rare gas and O₂;generating a plasma of the processing gas and concurrently applying abias by applying a high frequency power for both plasma generation andbias to the lower electrode; and applying a DC voltage to the upperelectrode.
 3. The plasma etching method of claim 1, wherein the hardmask layer is an amorphous carbon film.
 4. The plasma etching method ofclaim 2, wherein the hard mask layer is an amorphous carbon film.
 5. Theplasma etching method of claims 1, wherein the C_(x)F_(y) gas is C₃F₈ orCF₄.
 6. The plasma etching method of claim 2, wherein the C_(x)F_(y) gasis C₃F₈ or CF₄.
 7. The plasma etching method of claim 1, wherein theC_(x)F_(y) gas is C₃F₈, and the flow rate thereof is equal to or greaterthan the flow rate of C₄F₈.
 8. The plasma etching method of claim 2,wherein the C_(x)F_(y) gas is C₃F₈, and the flow rate thereof is equalto or greater than the flow rate of C₄F₈.
 9. The plasma etching methodof claim 1, wherein the absolute value of the DC voltage ranges fromabout 800 to 1200 V.
 10. The plasma etching method of claim 2, whereinthe absolute value of the DC voltage ranges from about 800 to 1200 V.11. The plasma etching method of claim 1, wherein the rare gas is Ar orXe.
 12. The plasma etching method of claim 2, wherein the rare gas is Aror Xe.
 13. The plasma etching method of claim 1, wherein a hole having awidth of about 70 to 90 nm and an aspect ratio of about 1:15 to 1:25 isformed by the plasma etching.
 14. The plasma etching method of claim 2,wherein a hole having a width of about 70 to 90 nm and an aspect ratioof about 1:15 to 1:25 is formed by the plasma etching.
 15. Acomputer-readable storage medium for storing therein acomputer-executable control program for controlling a plasma etchingapparatus including a vacuum-evacuable processing chamber having thereina lower electrode serving as a mounting table for a substrate and anupper electrode disposed to face the lower electrode, the plasmaprocessing apparatus performing a plasma etching by converting aprocessing gas supplied in the processing chamber into a plasma by meansof applying a high frequency power of a relatively high frequency levelfor plasma generation to the upper or the lower electrode; applying ahigh frequency power of a relatively low frequency level for bias to thelower electrode; and also applying a DC voltage to the upper electrode,wherein, when executed, the control program controls the plasma etchingapparatus to perform the plasma etching method of claim
 1. 16. Acomputer-readable storage medium for storing therein acomputer-executable control program for controlling a plasma etchingapparatus including a vacuum-evacuable processing chamber having thereina lower electrode serving as a mounting table for a substrate and anupper electrode disposed to face the lower electrode, the plasma etchingapparatus performing a plasma etching by converting a processing gassupplied in the processing chamber into a plasma by means of applying ahigh frequency power for both plasma generation and bias to the lowerelectrode; and also applying a DC voltage to the upper electrode,wherein, when executed, the control program controls the plasma etchingapparatus to perform the plasma etching method of claim 2.